An important tool in the analysis of fluids is diagnostic test strips. Diagnostic test strips have a test pad that incorporates a reagent capable of changing color when contacted by a predetermined constituent in a test sample. The intensity and degree of the color change is correlated to the concentration of the predetermined constituent in the test sample. The absence of a color change indicates that the predetermined constituent is not present in the sample, or at least is present below detectable levels.
Diagnostic test strips are available to assay blood, urine, and other body fluids for a predetermined constituent. Diagnostic test strips also are used to determine the presence and/or concentration of constituents in such diverse fluids as water and wine. The test strips utilize different reagents to selectively respond to different predetermined constituents. Accordingly, test strips are available to assay for various constituents, such as those found in urine. The urine assay results allow medical personnel to diagnose various disease states and to institute a proper therapy.
It is important to accurately measure the concentration of an unknown or analyte in a test sample because an inaccurate measurement can lead to an erroneous interpretation of the test result. Investigators, therefore, continually strive both for test strips that eliminate, or at least minimize, the occurrence of false positive and false negative assay results, and for detection apparatus that accurately determine the color of the test strip such that the color change of the strip can be properly correlated to the concentration of the unknown or analyte in the test sample.
In a typical analysis using a test strip, the test strip is dipped in a test sample (such as urine), excess test sample is blotted from the test strip, and the test pad is examined for a color change. The color change of a test strip can be determined by simple observation using the human eye, wherein comparison of the color change to a standardized chart allows correlation of the color change to an unknown or analyte concentration.
Such visual observation is suitable for some assays, but other assays require a more sophisticated detection and measurement method. Therefore, for various assays using a test strip, it is useful to utilize a detection apparatus, like a spectrophotometer, to detect and measure the color change resulting from contacting the test strip with a test sample.
In urinalysis, a conventional spectrophotometer determines the color change resulting from disposing a urine sample on a test pad. The test strip then is placed at a designated location in the spectrophotometer, and a start button is pressed which causes the spectrophotometer to automatically process and inspect the reagent strip. The spectrophotometer illuminates the pad and takes a number of reflectance readings from the pad, each having a magnitude relating to a different wavelength of visible light. The color of test pad then is determined from the relative magnitudes of red, green, and blue reflectance signals.
Conventional spectrophotometers can be used to simultaneously perform a number of different urinalysis tests by utilizing a test strip on which a number of different reagent pads are disposed. Such a reagent strip, like MULTISTIX.RTM., available from Bayer Corporation, Elkhart, Ind., can have ten different reagent pads for assaying ten different analytes. Each reagent pad incorporates a different reagent which results in a color change in response to the presence of a different constituent in urine, such as leukocytes (white blood cells) or red blood cells.
For example, a spectrophotometer has been used to detect the presence of red blood cells in a urine sample. Red blood cells present in the urine react with the reagent incorporated into a test pad, causing the test pad to change color to a degree related to the concentration of red blood cells in the urine sample. In the presence of a relatively large concentration of red blood cells, the test pad changes in color from yellow to dark green.
As stated above, a conventional reflectance spectrophotometer detects the concentration of red blood cells in urine by illuminating the test pad and detecting, via a conventional reflectance detector, the amount of light received from, i.e., reflected from, the test pad. The reflected light is related to the color of the reagent pad. Based upon the magnitude of the reflectance signal generated by the reflectance detector, the spectroscope assigns the urine sample to one of a number of categories, e.g., a first category corresponding to no blood (negative), a second category corresponding to a trace concentration (lysed or intact), a third category corresponding to a small blood concentration, a fourth category corresponding to a medium blood concentration, and a fifth category corresponding to a high blood concentration. From the assay, a diagnosis is made, and a treatment regimen is instituted.
A problem encountered when using a spectrophotometer to measure, or read, the color change of a test pad is an inaccurate assay, or a false negative assay result attributed to an improper alignment of the test strip in the spectrophotometer. Typically, a detection apparatus requires some type of optical positioning or calibration relative to the position of the test strip being read by the detection apparatus. Optical positioning of the test strip presently ig accomplished by: (1) controlling the physical position of the strip either by attaching the strip to a roll, confining the strip in a track, or moving the strip along rails, and/or (2) by checking for predetermined reagents by measurement of a visible color.
A major disadvantage with the above approaches is that they rely on medical laboratory personnel or instrument mechanics to accurately position the test strip with respect to the optics of the detection device. These methods are not fail-safe, and misalignment occurs relatively frequently. Such misalignments result in erroneous analyte assays, but are neither recognized nor recorded as assay errors.
Relying solely on laboratory personnel or instrument mechanics without a fail-safe mechanism for proper alignment of a test strip in a detection apparatus can result in more than 1 in 100 strips being read incorrectly. Checking the pad for a visible color increases reliability, but measuring pad color attributable to a reagent whose color is dependent on the amount of analyte present in a specimen is limited. For example, a 0.200 inch wide test pad has to be mispositioned by more than 0.110 inch before the fail-safe mechanism is activated, and the detection apparatus rejects the test strip, i.e., does not measure the color of the strip until the strip is properly aligned. Furthermore, as illustrated in the following Table 1, false results occur at a mispositioning of as low as 0.055 inch.
The accuracy of the assays can be further improved by attaching the test strips to fixed positions on a test strip carrier prior to reading of the strip by the detection apparatus. However, this method reduces the ability of the apparatus to handle a variety of different types of test strips, and only is economical in apparatus that read high volumes of identical types of test strips.
TABLE 1 Effect of Strip Misposition in Spectrophotometer.sup.1) Expected Analyte Result Observed Result Glucose 100 mg/dL.sup.2) 100 mg/dL 100 mg/dL 100 mg/dL 0 mg/dL 0 mg/dL Ketone Trace Trace 0 mg/dL NM 0 mg/dL 0 mg/dL 15 mg/dL 15 mg/dL 15 mg/dL 15 mg/dL 0 mg/dL 0 mg/dL 40 mg/dL NM NM NM 0 mg/dL Protein Trace Trace 0 mg/dL NM 0 mg/dL 0 mg/dL 30 mg/dL NM NM NM NM 0 mg/dL 300 mg/dL 300 mg/dL 300 mg/dL 100 mg/dL NM 0 mg/dL Urobilinogen 1.0 mg/dL 1.0 mg/dL 0.2 mg/dL 0.2 mg/dL NM 0.2 mg/dL Bilirubin Small Small 0 mg/dL 0 mg/dL NM 0 mg/dL Strip.sup.3) 0" (inch) 0.05" 0.06" 0.07" 0.08" 0.10" Misposition (in inches) .sup.1) Spectrophotometer was a CLINITEK .RTM. 50, available from Bayer Corporation, Elkhart, IN, which relies on the user to align the test sample in a track, and utilizes a pad check for a visual color as a fail-safe. No strips were rejected in this test by the pad position check; .sup.2) mg/dL is milligrams per decilites; .sup.3) strips were 0.200 inch wide; and .sup.4) NM-not measured.
The results summarized in Table 1 demonstrate that a strip can be mispositioned by up to 0.110 inch, and still not be rejected by the position checks utilized in the detection apparatus. At a mispositioning of greater than 0.055 inch, some low-level positive assays were reported as false negative results. By mispositioning by 0.110 inch, almost all assays, even at moderate positive levels, were reported as false negative. The current positioning method uses the inherent color of the test pads as a check, but sensitivity is very low because a large tolerance is required due to differences in reagent reactivity with varying positive analyte levels. The present invention is directed to overcoming the problem of misalignment of a test strip in a detection apparatus, and providing a more accurate test strip assay.